1. Field of the Invention
This invention relates to a dual purpose, vehicle-mounted, vibrator for use in seismic exploration that can inject either compressional waves or shear waves into the earth using the same vibrator assembly.
2. Description of the Prior Art
As is well known in seismic surveys, large vibrators mounted on heavy-duty vehicles inject into the earth, a swept-frequency signal ranging from 5 to 180 Hz. These signals propagate downwardly, impinge upon various rock layers, and are reflected back to the surface. Seismic sensors coupled to the ground receive the reflected signals and convert them into electrical impulses that may be recorded for archival storage.
In general, seismic waves from a physical disturbance propagate through the earth in two modes. One mode is represented by a compressional or pressure (P) wave, wherein particle motion is parallel to the wave propagation direction. The second mode is a shear (S) wave, wherein particle motion is transverse to the wave propagation direction. Shear waves generated at the earth's surface may represent either motion parallel to the plane of the earth's surface or perpendicular thereto. In geophysical exploration, however, only shear waves generated parallel to the plane of the surface (S.sub.H) are of use. In this description it will be assumed that all references to shear waves are restricted to S.sub.H or "horizontal" shear waves.
Both P and S wave types are used in seismic surveying. P-waves penetrate to great depths, but lose resolution of the thinner beds. S-waves on the other hand better resolve thin strata, but have limited penetration. A comparison of P-wave and S-wave data is a useful aid in determining certain rock characteristics.
The shaker assembly of a vibrator usually comprises an inertia mass of several hundred to several thousand pounds. The mass includes an internal cylinder within which is mounted a reciprocable piston. At least one piston rod interconnects the piston with a ground contacting base plate. A well known hydraulic servo valve introduces high pressure hydraulic fluid alternately to opposite sides of the piston within the inertia-mass cylinder. Reciprocation of the piston and piston rod relative the inertia mass causes the base plate to shake the ground. It should be understood that the axis of motion of the shaker assembly is mounted vertically for P-wave generation. See for example U.S. Pat. No. 3,306,391 issued to Bays, Feb. 28, 1967.
S-waves are induced in a manner similar to that for P-waves except the axis of motion of the shaker assembly is oriented horizontally, hence the reciprocating motion is also horizontal. The shaker assembly is rigidly coupled horizontally between supports mounted on a base plate as taught by Clynch et al. (U.S. Pat. No. 3,159,233).
For the purpose of this disclosure, a vibrator assembly consists of a shaker, a base plate and supports therefor, a lifting mechanism and necessary compliant isolator means. A vibrator assembly is mounted to the chassis of a heavy-duty vehicle. A portion of the weight of the vehicle is used to apply a bias force to the vibrator base plate to prevent decoupling from the ground during operation. Usually the vibrator assembly is mounted on the vehicle by hydraulic jacks so the base plate can be lowered to the ground, or raised to clear the ground for transport. Compliant means such as airbags couple the base plate to the hydraulic jacks. The compliant means isolate the vehicle from vibrations caused by the reciprocating base plate, yet allow the bias force of the vehicle to be applied to the base plate.
Heretofore it was necessary to provide separate shaker assemblies, one for P-wave and one for S-wave generation. Because the shaker assemblies are very heavy, separate vehicles were usually required to handle them.
It is the purpose of this invention to provide a single, dual-mode vibrator/shaker assembly that can be carried by a single vehicle and that can be used to generate either P-waves or S-waves at the option of the user.